WHILE THE new Mexican plan is not a sweeping solution for all of the Latin debts, it is a highly useful and imaginative way to deal with some of them. It is important as a signal of the banks' responsiveness to Mexican concerns -- and, equally, of Mexican flexibility in meeting some of the bankers' anxieties.

Through its strong export performance, Mexico has earned substantial financial reserves. For some time the Mexicans have been considering how they might best use these reserves to reduce the country's burden of foreign debt. Working with the Morgan Guaranty Trust Co. in New York, and with the cooperation of the U.S. Treasury, the Mexicans developed an ingenious and attractive proposal to lay before their creditors.

Mexico will use $2 billion of its reserves to buy U.S. securities that, through the magic of compound interest, will be worth $10 billion in 20 years. They will then use those securities as collateral to guarantee repayment of $10 billion in new Mexican bonds, to be sold to banks at auction. A bank might offer to wipe out, say, $18 of the present unsecured debt in return for $10 in guaranteed and saleable bonds. If it actually works out that way, Mexico will extinguish $18 billion of bank debt for an investment of $2 billion plus interest over the next 20 years on the bonds. It will all be entirely voluntary. No bank will write off more than it chooses.

Not all banks will want to take advantage of this offer. The big international banks with their very long-term interests in Latin America will probably choose to hold their loans. But many smaller banks have wanted for years to get out of foreign lending, even at the cost of writing off a substantial part of the money owed to them.

Since Mexico's debt is $100 billion, this plan won't eliminate it. But the reduction will be significant while, on the other side of the equation, the growth of the Mexican economy will make the remaining debt easier to carry.

As for a sweeping, comprehensive solution to the Latin debts, there is no such thing in the real world. What's needed is a lot of special solutions like this one, tailored to specific circumstances. People sometimes say portentously that the Latins will never pay off their debts. That's true in the same sense in which the United States will never pay off its national debt. But paying off the debts is not necessary to building healthy economies. For the Latins it's necessary only to get debt service down to a tolerable ratio to the size of their economies and to their export earnings. The Mexican plan is a notable contribution to that purpose, and a highly promising model of cooperation between borrowers and lenders.